Device and method for load balancing for data packet transmissions in wireless networks

ABSTRACT

For load balancing in a large-scale wireless mesh network, a device, a system and a method are provided for controlling data packet transmissions in the wireless mesh network, wherein answers to more than one received data packet are aggregated in one answer-batch.

FIELD OF THE INVENTION

The invention relates to a device, a system and a method for controllingdata packet transmissions in a wireless network.

BACKGROUND OF THE INVENTION

Recently, wireless mesh networks attract more and more attention, e.g.for remote control of illumination systems, building automation,monitoring applications, sensor systems and medical applications. Inparticular, a remote management of outdoor luminaires, so-calledtelemanagement, becomes increasingly important. On the one hand, this isdriven by environmental concerns, since telemanagement systems enablethe use of different dimming patterns, for instance as a function oftime, weather conditions and season, allowing a more energy-efficientuse of the outdoor lighting system. On the other hand, this is alsodriven by economical reasons, since the increased energy efficiency alsoreduces operational costs. Moreover, the system can remotely monitorpower usage and detect lamp failures, which allows determining the besttime for repairing luminaires or replacing lamps.

Current radio-frequency (RF) based wireless solutions use either a starnetwork topology or a mesh network topology. In a star network, acontroller has a direct communication path to every node in the network.However, this typically requires a high-power/high-sensitivitybase-station-like controller to be placed at a high location (e.g. ontop of a building), which makes the solution cumbersome to deploy andexpensive. In a mesh network, the plurality of nodes does in general notcommunicate directly with the controller, but via so-called multi-hopcommunications. In a multi-hop communication, a data packet istransmitted from a sender node to a destination node via one or moreintermediate nodes. Nodes act as routers to transmit data packets fromneighboring nodes to nodes that are too far away to reach in a singlehop, resulting in a network that can span larger distances. By breakinglong distances in a series of shorter hops, signal strength issustained. Consequently, routing is performed by all nodes of a meshnetwork, deciding to which neighboring node the data packet is to besent. Hence, a mesh network is a very robust and stable network withhigh connectivity and thus high redundancy and reliability.

In the prior art, mesh network transmission techniques can be divided intwo groups: flooding-based and routing-based mesh networks. In aflooding-based mesh network, all data packets are forwarded by all nodesin the network. Therefore, a node does not have to make complicatedrouting decisions, but just broadcasts the data packet. By these means,the technique is quite robust. However, in large networks, the dataoverhead due to forwarding impacts the overall data rate. Moreover,collisions of data packets are more likely to occur, further reducingthe overall performance. Hence, the main problem of this solution is thescalability. Routing-based mesh networks can be further divided intoproactive and reactive schemes. In proactive routing-based meshnetworks, all needed network paths are stored in routing tables in eachnode. The routing tables are kept up to date, e.g. by sending regularbeacon messages to neighboring nodes to discover efficient routingpaths. Although the data transmission is very efficient in such kind ofnetwork, the scalability is still low, since in big networks, theproactive update of the routing tables consumes large parts of networkresources. Moreover, the routing tables will grow with the scale of thenetwork. In addition, the setup of the network requires time andresources in order to build up the routing tables. Reactive schemes, incontrast, avoid the permanent overhead and large routing tables bydiscovering routes on demand. They use flooding to discover networkpaths and cache active routes or nodes. When routes are only usedscarcely for single data packets, flooding the data packets instead ofperforming a route discovery might be more efficient. If routes are keptlong enough to avoid frequent routing, reactive schemes degenerate toproactive schemes. An example for a reactive routing-based mesh networkis used in ZigBee. However, the main problem of this protocol scheme isstill the scalability of the network.

Thus, most transmissions in wireless mesh networks are performed in amulti-hop mode. Since then every data packet or message is transmittedmultiple times due to the forwarding, thereby reducing the overallnetwork throughput, the network scalability of wireless mesh networks isvery limited. Also, data packet collisions are more likely to occur,further degrading the overall performance. In particular, whennode-initiated data traffic, e.g. statistics report data or othertime-uncritical data, is transmitted by a plurality of nodes to a datacollector node or control center choosing nearly the same moment intime, an overload problem will arise, causing data collision and loss.

Moreover in non-reliable networks, such as RF networks, a data packetcan also get lost during transmission for other reasons than collisions,e.g. due to network overload or deteriorated link quality.Retransmissions can reduce the likelihood, but never can guarantee asuccessful transmission. The likelihood of packet losses adds up, when apacket has to travel over multiple hops. In large-scale multi-hopnetworks, the number of hops a data packet has to travel iscomparatively large. For instance, in a large RF telemanagement systemcomprising thousands of nodes, 20-40 hops are likely to occur. Hence,the delivery chance of an individual data packet decreases with its hopdistance, since with every hop, there is a chance that the data packetgets lost. Moreover, due to congestions and temporary errors at routinglevel, the likelihood of packet losses in the multi-hop scenarioincreases further. This makes data acknowledgment at transport orapplication layer necessary, if delivery guarantees are required by theapplication. The drawback of an acknowledge mode, however, is that dataacknowledgements increase the data load in the network and theexperienced delay increases significantly, especially when data packetshave to be retransmitted at transport or application layer. Moreover,when a multicast or broadcast data packet is responded to (acknowledged)by many or all receiver nodes within a short period of time, a so-calledacknowledgement storm may occur, causing an overload problem in theneighborhood of the sender node and thus in data collision and loss.This can be avoided in globally scheduled networks, where every node hasfix individual timeslots for transmissions. But this global scheduleneeds global coordination and configuration, thus involving a cumbersomeset-up procedure. Furthermore, sending over a global schedule andcreating a global schedule for all nodes create a high data overhead andmanagement overhead, respectively. Moreover, when the slots are usedonly infrequently, this decreases drastically the possible bandwidth.Consequently, this approach is also not suitable for large-scalenetworks.

Hence, a big disadvantage in common wireless networks is constituted onthe one hand due to the tedious deployment and configuration and on theother hand by the very limited network scalability. Especially, RFtelemanagement networks suffer from significant overload due to theirtopology and size, particularly at local level, which limits theirscalability. This occurs because messages are generated and transmittedby layers of the communication stack without considering the networktopology. Improving the success and reliability of transmissions istherefore crucial in large-scale networks, such as street illuminationsystems with a high number of luminaire nodes. This is becauseend-to-end retransmissions typically occurring at the higher layers inthe communications stack are far more costly and delay intensive.Consequently, efficient routing protocols are required for large-scalewireless networks in order to minimize data packet collisions andsubsequent data loss and to achieve the required throughput, responsetimes and robustness.

US 2006/0187836 A1 describes a communication device that delays ordiscards time-insensitive traffic prior to time-critical traffic byintroducing a virtual bottleneck queue.

Whetten et al. (IEEE Network, vol. 14, no. 1, pages 37-47, XP001195288)describes the Reliable Multicast Transport Protocol II (RMTP-II). Areceiver node periodically informs its parent node about the packets ithas or has not received by unicasting a TRACK (hierarchical ACK) packetto its parent node. Each parent node aggregates the TRACKs from itschild nodes and unicasts a single aggregated TRACK to its parent (top)node. The top node aggregates the TRACKS from its child nodes andunicasts a single TRACK to the sender node.

US 2010/246600 A1 describes a system for reducing overhead in acommunication system. Different frame types are aggregated into anenhanced aggregated frame in response to a determination that thedifferent frames are suitable for transmission within a singlereservation period. If the frames are time-sensitive, the transmitter,or transceiver, will not aggregate the frames as it needs to flush outthe frames as quickly as possible.

US 2008/137624 A1 describes a global wireless sensor networkarchitecture and protocol for remote supervision, asset control andoperational management of ad hoc networks. An application of thistechnology is the remote control and supervision of lighting systemsfrom centralized regional operations centers.

SUMMARY OF THE INVENTION

In view of above disadvantages and problems in the prior art, it is anobject of the present invention to provide a device, a system and amethod for transmitting data packets in a wireless network that solvethe overload problem and provide a high level of scalability andtransmission reliability, without requiring high processing power ineach network node.

The object is solved by the features of the independent claims.

The invention is based on the idea that the message flow towards and/orfrom a data collector node is scheduled and/or shaped to balance thetransmission load across time and/or network regions. Preferably, thisis achieved in a cross-layer approach, wherein network components andprotocols can make use of information available from each other, e.g. bymeans of cross-layer communication, thus relieving the data load in theneighborhood of a data collector node or control center and increasingthe network scalability.

In one aspect of the present invention, a device is provided forcontrolling data packet transmissions in a wireless network. The devicepreferably comprises a control unit that is configured to aggregateanswer data packets into one answer-batch, when more than one datapacket is received. By these means, the device according to the presentinvention is adapted to optimize the data packet response. Thus,redundant headers can be omitted and data overhead can be reduced, sothat the payload of the transmission can be increased, thus savingnetwork resources. Hence, the device according to the present inventionschedules and/or shapes data traffic when responding to a received datapacket in order to balance the transmission load across time and networkregions.

In order to be aggregated in the one answer-batch, answers to thereceived data packets may be postponed, respectively. Preferably, thedetermined answer interval is considered so that the answer batch istransmitted within the determined answer interval. When it is determinedthat the received data packets are received from a single sender, theanswer-batch is unicasted to the one sender. Similarly, when it isdetermined that the received data packets are received from a pluralityof senders, the answer-batch may be a multicast or a broadcast to morethan one of the plurality of sender nodes. Thus, in one embodiment ofthe present invention, the control unit may aggregate answer datapackets for an individual node and/or for a predetermined group of nodesin order to avoid unnecessary data overhead. In particular, if theanswer data packets correspond to acknowledgements, which contain almostno payload, but still need the full network layer and MAC layer header,data overhead for a transmission is significantly reduced and networkresources are saved. In case of forming an answer-batch for apredetermined group, the group of nodes may be determined appropriately,for instance based on network efficiency or on a distance between therespective nodes. However, it is also possible to determine the group ofnodes based on their location within the network or on their networkaddresses or the like. Alternatively, a plurality of appropriate groupsmay be predefined and stored in each node.

Preferably, the device can be added or coupled to at least one of anetwork node, a data collector node and a control center. Thus, thedevice is associated with a network node, which may also be a datacollector node. The data collector node may be any node that isconfigured to communicate with a control center of the network and mayfunction as a kind of gateway. For instance, the device may be adaptedto be inserted in an existing circuit board or to be connected to anexisting controller of the node. In particular, when the device is addedto existing network components, existing wireless networks as streetlighting systems can be upgraded to the inventive concept. In additionto the control unit, the device may further comprise a memory and/or atransceiving unit for receiving and transmitting data packets.

The wireless network may have a mesh topology. Furthermore, the nodes ofthe wireless network may be stationary and the positions of the nodeswithin the wireless network may be known. In the network, data packettransmission may be performed as multi-hop transmissions either inhop-by-hop acknowledge mode with acknowledgements for every hop or inend-to-end acknowledge mode. Thereby, successful data packettransmission is verified and unnecessary data packet retransmissions areavoided.

In a preferred embodiment, the received data packet is a multicast orbroadcast data packet that is sent to more than one receiver, e.g. arequest for data delivery. The answer may be a time-uncritical responsedata packet and/or an acknowledgement of the received data packet. Thus,the device according to the present invention may be adapted to optimizethe data packet acknowledgement by aggregating a plurality ofacknowledgements into one answer-batch. This way, a highly efficientacknowledged mode can be realized without creating large data overheadsor causing an increased number of data collisions.

In one embodiment, the control unit of the device can differentiatebetween time-critical and time-uncritical data packets. For instance,the control unit performs an analysis step in order to determine whetherthe answer data packet is a time-critical or a time-uncritical datapacket. By these means, it can be assured that time-critical datapackets are transmitted without delay or even prioritized with respectto time-uncritical data packets. In addition, time-uncritical datapackets will not block the transmission of time-critical data packets.

In a further preferred embodiment, the control unit is adapted torandomly select a time slot within a determined answer interval fortransmitting an answer data packet to a sender of a received datapacket. In particular, if the received data packet is a multicast orbroadcast data packet, this helps to avoid data packet collisions at thesender, when some or all of the receivers having received the multicastor broadcast data packet respond with very short time lags. Forinstance, the received data packet may be a request for data delivery,upon which the receivers transmit the requested data as an answer. Thereceiver and the sender, respectively, can be any of the control center,the data collector node or the nodes. The determination of the answerinterval is preferably performed by the control unit of the device. Theanswer interval may be specific for the received data packet. Forinstance, if the answer data packet is a time-uncritical data packet,the answer interval may be determined to be longer than an answerinterval for a time-critical answer data packet. The determination ofthe answer interval may also be based on a number of receivers of thereceived data packet, if the received data packet is a multicast orbroadcast data packet. Additionally or alternatively, the determinationof the answer interval may be based on a number of receivers of theanswer data packet, i.e. on the number of nodes, to which the datapacket is transmitted. Furthermore, the answer interval may bedetermined based on network characteristics, such as networkcapabilities, network topology, current network load and expectednetwork load. Possibly, a schedule is stored in the device and thecontrol unit can determine the answer interval by means of thisschedule. Further additional or alternative criteria for determining theanswer interval may be information included in the received data packet,e.g. network load determined or estimated by the sender, a deadlinedetermined by the sender, a time stamp indicating a point in time, whenthe sender has generated or sent the data packet, etc. When the senderdefines a deadline, before which the answer data packet has to be sent,the sender decides, when the answer has to be expected at the latest.This allows a sender to better adjust a retransmission timer. Thedeadline should define an answer interval longer than the time normallyneeded for receiving the data packet and transmitting the answer datapacket. In case that a multicast data packet has to be answered, theprobability of data collisions and subsequent data loss at the senderdecreases with longer answer intervals.

In addition, the control unit may be adapted to distribute the networkload over different network regions by scheduling the answers fromnetwork nodes to a data request message based on at least one of a hopdistance, a network area and a network address of the respective node.In a preferred embodiment, when the device is associated with a nodehaving a low hop distance to a data collector node, the control unit ofthe device may schedule data packet transmission to be prior than datapacket transmission of a device in a network area with high hop distanceto the data collector node. Thus, the device can shape data trafficconsidering the network topology. This way, it is ensured that datapackets to be transmitted by a node in a low hop distance area do notpile up, so that nodes in the low hop distance area can serve as routersforwarding data packets from nodes in areas with higher hop distances.Thus, the load for nodes in the low hop distance area is reduced.

Furthermore, the control unit may be capable of setting back-off delaysbased on network topology, such as a spatial or geographical distancebetween the device and a data collector node, a hop distance of thedevice to a data collector node, a location of the device within thenetwork, a network address associated with the device or a combinationthereof. The back-off delay may refer to back-off delays of dataretransmission introduced by a MAC layer or a network layer. Preferably,a back-off delay in a device close to the data collector node is shorterthan the back-off delay in a device far from the data collector node.

In a preferred embodiment of the present invention, the device is usedin a control network for telemanagement of a lighting system, e.g. astreet lighting system. By means of such a telemanagement system,luminaire nodes may be easily switched on/off and/or the dimming patternthereof may be controlled based on parameters, such as day-time, season,weather, ambience brightness, occurrence of traffic accidents, presenceof road works, etc. These parameters may be determined by sensorsprovided with the luminaire nodes and may then be reported to a controlcenter.

In a further aspect of the present invention, a system is provided forcontrolling data packet transmission in a wireless network, the systemcomprising a control center and a plurality of nodes, wherein thecontrol center and/or at least one of the nodes comprises the deviceaccording to one of the above-described embodiments. Preferably, datapacket transmission is performed via multi-hop transmissions in thewireless network. In one embodiment, the nodes are associated withluminaires of an outdoor lighting system. Moreover, data packettransmission from a node to a data collector node or to a control centermay be scheduled based on at least one of a location of the node withinthe network, a network address of the node and a hop distance of thenode to the data collector node or to the control center. Preferably,the scheduling is performed by the device located in a node, so that thenode may report to a data request message multicasted by a controlcenter or by a data collector node with a node-specific delay.Additionally or alternatively, a node of the network may be polled by acontrol center or a data collector node based on its network locationand/or on the hop distance to the data collector node or to the controlcenter. The polling may be performed at a network layer, transport layeror application layer of the control center or the data collector node orby a device included therein. Then, after the polling, the nodes reportat different moments in time, thus avoiding data packet collision.

In another aspect of the present invention, a method is provided forcontrolling data packet transmissions in a wireless network, whereinpreferably answer data packets to more than one received data packet areaggregated in one answer-batch. By these means, data traffic in awireless network can be scheduled and/or shaped with respect totransmission times and network regions in order to avoid networkoverload and data loss.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1A illustrates an example of a wireless mesh network;

FIG. 1B illustrates a schematic view of a device according to thepresent invention;

FIG. 2 illustrates multicasting;

FIG. 3A illustrates a flow diagram for optimized data transmissionaccording to one embodiment of the present invention;

FIG. 3B illustrates a flow diagram for optimized data transmissionaccording to a modification of the embodiment shown in FIG. 3A;

FIG. 4 illustrates a flow diagram for optimized data transmissionaccording to another embodiment of the present invention;

FIG. 5A illustrates a spatial distribution of nodes in an outdoorillumination telemanagement system; and

FIG. 5B shows a schematic view of a wireless mesh network illustratinghop-distances of nodes to a data collector node.

DETAILED DESCRIPTION

Preferred applications of the present invention are actuator networks,sensor networks or lighting systems, such as outdoor lighting systems(e.g. for streets, parking and public areas) and indoor lighting systemsfor general area lighting (e.g. for malls, arenas, parking, stations,tunnels etc.). In the following, the present invention will be explainedfurther using the example of an outdoor lighting system for streetillumination, however, without being limited to this application. In thefield of lighting control, the telemanagement of outdoor luminaires viaradio frequency network technologies is receiving increasing interest,in particular solutions with applicability for large-scale installationswith segments of above 200 luminaires.

In FIG. 1A, a typical network with mesh topology is shown. A pluralityof nodes 10 (N) is connected to each other by wireless communicationpaths 40. Some of the nodes 10 function as data collector nodes 50(N/DC), which receive data packets from the surrounding nodes 10 viasingle-hop or multi-hop transmissions and transmit them to a controlcenter 60 and vice versa. Thus, the data collector nodes 50 may operatein the manner of gateways between the nodes 10 and the control center60. The wireless communication paths 40 between the nodes 10 and datacollector nodes 50 may be constituted by RF transmissions, while theconnection 70 between the data collector nodes 50 and the control center60 may make use of the Internet, mobile communication networks, radiosystems or other wired or wireless data transmission systems.

In a telemanagement system for outdoor lighting control, communicationis very asymmetric. Most of the traffic is generated by the nodes 10,e.g. reporting their state, their dimming profile, sensor values orpower usage to the control center 60. The other traffic consists ofcontrol commands from the control center 60 to the different nodes 10,e.g. for adjusting a dimming pattern or switching on/off lamps.Therefore, most traffic is constituted by N:1 traffic (unicasts),whereas the traffic from the control center 60 to the nodes 10 consistsof 1:N traffic, either in unicast, multicast or broadcast mode.Moreover, the number of luminaire nodes 10 is extremely high in anoutdoor lighting system such as a street lighting system. Hence, thesize of the network is very large, especially when compared to commonwireless mesh networks, which typically contain less than 200 nodes. Inaddition, the nodes 10 typically have limited processing capabilitiesdue to cost considerations, so that processing and memory resources inthe luminaire nodes 10 will be limited. Thus, communication protocolsfor transmitting data packets between single nodes 10 should considerthe limited resources for efficient and fast data packet transmission.Furthermore, compared to other so-called ad-hoc mesh networks, thetelemanagement system for an outdoor lighting control network isstationary, i.e. the nodes 10 do not move. Also, all nodes 10 may beconnected to mains power. Consequently, network changes will be mainlydue to a changing environment, e.g. due to traffic. If the nodes 10 arestationary, the physical positions of the nodes 10, for instance GPScoordinates, may be known in the system, enabling geographic orposition-based routing. Furthermore, telemanagement of an outdoorlighting system does not require a high data rate. That means that alarge part of the data traffic consists of time-uncritical data packets,e.g. status report data, statistical data, sensor data or the like.However, there are some scenarios, where a low response time is neededfor a certain type of messages or data packets (time-critical datapackets). For instance, when a traffic accident is detected, nodes 10 ofthe corresponding area can be controlled as to immediately switch tofull power.

In FIG. 1B, a device 100 according to the present invention is shown.The device 100 can be associated with a node 10, a data collector node50 or the control center 60 of a wireless multi-hop mesh network. Thedevice 100 comprises a control unit 200. Moreover, either the node 10,the data collector node 50 or the control center 60, respectively, orthe device 100 comprises a transceiving unit 300 for transmitting orreceiving data packets via wireless communication paths 40, e.g. via RFtransmission. Since RF transmissions do not require high transmissionpower and are easy to implement and deploy, costs for setting up andoperating a network using the device can be reduced. This is especiallyimportant for large RF networks, e.g. a RF telemanagement network forlighting systems. However, the data packet transmission mayalternatively use infrared communication, free-space-visible-lightcommunication or powerline communication.

As shown in FIG. 2, data packet transmission from a sender A to severalspecific receivers B among the plurality of network nodes 10 can beperformed as a multicast (arrows). The sender A or at least one of thereceivers B may also refer to a data collector node 50 or the controlcenter 60. Preferably, however, data packet transmission from the datacollector node 50 to the respective luminaire nodes 10 is performed byflooding, wherein all data packets are forwarded by all luminaire nodes10 in the network. The data packet contains at least information aboutthe sender A and one or more destination nodes 10 or receivers B. Thedata packet is then decoded by the at least one destination node 10. Fordata packet transmission from the luminaire nodes 10 to the datacollector nodes 50, a routing-based solution is preferred. Preferably, aproactive routing structure is used, since the routes to the datacollector nodes 50 are regularly used. In the proactive routingstructure, a routing table is stored in every node 10, indicating whichneighboring node 10 can act as connection towards the data collectornodes 50. Thus, data packets can be sent to the closest data collectornode 50 in a very efficient and fast way. Advantageously, each node 10keeps information about multiple downlink neighboring nodes 10 asalternative routes in order to increase reliability. If one neighboringnode 10 is not reachable due to strong interference or complete failure,the routing protocol has then additional alternatives to route the datapacket to the data collector node 50.

Hence, in a wireless network, transmission of the same data packet tomultiple nodes 10 (groups or even all nodes 10) can be efficientlyachieved by broadcasting or multicasting the data packet (see FIG. 2).If a delivery guarantee is required in order to confirm a successfultransmission, the broadcast or multicast transmission was performed inacknowledged mode, i.e. a receiver B having received a data packetconfirms successful transmission by transmitting an acknowledgement(ACK) to a sender A. However, one big disadvantage of the acknowledgemode is that acknowledge data packets contain almost no payload, butstill need the full network layer and MAC layer header, thuscontributing to network overload. That means that the overhead of anacknowledgement data packet is very high compared to other data packetsand the transmission of acknowledgements fills therefore in many cases asignificant amount of the available payload. Hence, according to oneembodiment of the present invention shown in FIG. 3, multipleindependent acknowledgements can be aggregated into one answer-batch.

In FIG. 3A, an embodiment for aggregating several acknowledgements isshown for the case that the acknowledgements correspond to data packetsreceived from the same sender A. When a receiver B receives a datapacket from a sender A (S300), the receiver B waits for a predeterminedtime (S301), whether more data packets are received. This makes it morelikely that an aggregation takes place. Thus, the receiver B postponesthe acknowledgements to data packets received during this predeterminedtime and stores the respective acknowledgements (S302). After thepredetermined time has passed, the receiver B determines, whetherseveral data packets have been received from the same sender A. In casethat several data packets are received from the same sender A within thepredetermined time interval (S303), acknowledgements for these datapackets are assembled in S304 into one answer-batch oracknowledgement-batch (ACK-batch). The ACK-batch is then transmitted orunicasted to the one sender A (S305).

In FIG. 3B, a similar procedure is illustrated for the case that severaldata packets are received from different senders A. After havingreceived a data packet (S300), the receiver B waits for a predeterminedtime interval (S301), whether more data packets are received, postponingand storing acknowledgements received during this time (S302). When itis determined that several data packets are received from differentsenders A within the predetermined time interval (S313), the receiver Bcan determine one or more appropriate groups of senders (S314). This maybe based on the received data packets, on the answer to be transmitted,on the location of the senders A within the network, on the hop distanceof the senders A to the receiver B or on network addresses of thesenders A. However, the step of determining an appropriate group (S314)may also be omitted, for instance, when groups of senders A arepredefined and stored in the receiver B. Finally, the acknowledgementsto received data packets of senders A of one appropriate group areaggregated into one answer-batch or ACK-batch (S315) and the ACK-batchis then transmitted or multicast to all senders A of this group (S316).Thus, the receiver B groups or sorts the senders A of the received datapackets. This is particularly useful, when the control center 60 or adata collector node 50 has to acknowledge a plurality of data packetssent from a plurality of nodes 10. Hence, depending on the underlyingcommunication protocol, it is possible to map such answer-batches tonetwork-efficient multicast groups. Possibly, not all senders A of thereceived data packets can be allocated to one of the appropriate groupsof senders. In this case, their acknowledgements may be sent separately.Furthermore, it may happen that the senders A of the received datapackets cannot be allocated all to the same appropriate group ofsenders. Then, several ACK-batches may be transmitted, one for eachgroup of senders.

In a modification of the above described embodiments, it is furtherdetermined, whether the received data packet involves a time-criticalanswer. In case that the answer data packet is time-uncritical, a muchlarger waiting period may be chosen, so that the likelihood of answeraggregation is increased. Hence, the predetermined waiting time may beadjusted based on a required response time. In case that differentanswer intervals are determined for the plurality of received datapackets, the shortest answer interval may be chosen. By these means,network resources can be even more efficiently saved.

The embodiments as described above and illustrated in FIGS. 3A and 3Bare not limited to the aggregation of acknowledgements. Headers may alsobe saved when transmitting other answer data packets by aggregatingseveral answer data packets into one answer-batch. It is pointed outthat the answer-batch as well as the acknowledgement batch comprises aplurality of independent answers or acknowledgements, respectively. Thisway, network resources are saved and data traffic is reduced.

A further problem occurring in case of multicasting or broadcasting inacknowledge mode consists in a so-called acknowledgement storm that mayoccur at the sender A, resulting in data collision and loss. Therefore,according to another embodiment of the present invention, dataacknowledgement is additionally scheduled randomly by each receiver B asshown in FIG. 4.

In FIG. 4, optimized data transmission according to another embodimentof the present invention is exemplarily illustrated for acknowledgementtransmission. After a receiver B has received a multicast or broadcastdata packet from a sender A (S40), each receiver B determines an answerinterval (S41). After determining the answer interval (S41), thereceiver B selects randomly a time slot within the answer interval (S42)and transmits an acknowledgement data packet to the sender A in theselected time slot (S43), confirming the successful receipt of thereceived data packet.

The answer interval may be determined based on the number of receiversB, to which the multicast or broadcast data packet was sent. Forinstance, if a data packet is sent to a large number of receivers B, theanswer interval may be determined to be longer than for a data packetsent to only a few receivers B. The number of receivers B may bepredetermined for a certain kind of data packets. For instance, whenusing a group address for the multicast data packet, the group addressmay refer to a predetermined group of receivers B with a predeterminednumber of nodes 10. In particular, when a data request message is amulticast or a broadcast by a data collector node 50 or the controlcenter 60, each receiver node 10 may know the number of receivers B thatwill respond to the received data request. The answer interval may alsobe determined based on information included in the received data packet.Thus, the sender A may know or estimate the amount of receivers B in agroup and add this information to the multicast or broadcast datapacket. Additionally or alternatively, the sender A may estimate ordetermine a current or expected network load. Possibly, the answerinterval is determined to be similar for each receiver B. However, sinceevery receiver B randomly picks a time slot out of the answer intervaland transmits its answer data packet in the selected time slot, a datacollision at the sender node 10 becomes less likely. Of course, thelonger the answer interval or the lower the number of receiver nodes B,the less likely are data collisions. Thus, data loss is reduced andretransmissions can be avoided. Moreover, the predetermined time of stepS301 may be defined such that it is within a specified deadline oranswer interval as determined in S41. Possibly, the predeterminedwaiting time of S301 is chosen randomly within the answer interval.

In a further embodiment, the sender A transmits a deadline with the datapacket, when an answer data packet, e.g. an acknowledgement, has to betransmitted at latest. Then, the answer interval will be determined asthe time interval from the time of receiving the data packet and thepredetermined deadline. Each receiver B selects at random a time slotwithin this answer interval, i.e. before the deadline, and transmitsthen its answer data packet within the selected time slot.Alternatively, the received data packet comprises a time stamp, e.g.indicating the time of completing the data packet or transmitting thedata packet by the sender A, which is then used by each receiver B asreference in order to determine the answer interval. For instance, apredefined time interval may be stored in each node 10, e.g. dependingon the kind of received data packet, so that with the time stamp and thepredefined time interval, each receiver B can easily determine thedeadline, until which an answer data packet to the received data packethas to be sent. Thus, by including a deadline or a time stamp in thedata packet, so that the deadline and, thus, the answer interval can bedetermined, the sender A can shape the data traffic. Possibly, thesender A accounts for current or expected network load, when setting thedeadline. By specifying a deadline, the sender A can furthermoreoptimize the adjustment of a retransmission timer, since it decidesitself, when the last answer data packet has to be expected at latest.

Of course, the process shown in FIG. 4 is not limited to dataacknowledgement. In contrast, this process can be adapted for schedulingany response to multicast or broadcast data packets, e.g. answers tomulticast data request messages. By these means, a randomized schedulefor answering to received multicast data packets is created locally ateach receiver B in an ad-hoc manner, thus minimizing the probability ofa data collision in the network e.g. at the sender A.

In a further embodiment of the present invention, transmissions of datapackets, especially of time-uncritical data packets, are scheduledconsidering additionally network topology in order to minimize datacollisions and subsequent data packet losses. The transmission schedulemay be applied to N:1 or to 1:N communication patterns, e.g. tocommunication links from a data collector node 50 to the plurality ofnodes 10 or from the plurality of nodes 10 to the data collector node50. However, even though the communication from the nodes 10 to the datacollector node 50 or to the control center 60 is more efficient than inthe opposite direction, there is still room and need for improvement,because the biggest bulk of information to be communicated in a RFtelemanagement system will be statistical data or status reportinformation from the nodes 10. Therefore, this embodiment is preferablyadopted for N:1 communications from the nodes 10 to the data collectornode 50 or to the control center 60 and will thus be explained usingthis example. The transmission schedule may be based on at least one ofthe location of the senders A within the network, the hop distance ofthe senders A to a data collector node 50 and the network addresses ofthe senders A.

When the transmission schedule is based on the location of the senders Awithin the network, different areas may be defined such that thecommunication links 40 from nodes 10 of different areas do not interferewith each other. This avoids overloading of certain bottleneck areas ofthe network, in particular in the vicinity of a data collector node 50.Sender nodes 10 may find out about their location by consulting theirnetwork or MAC address. The transmission schedule may be established anddistributed by the control center 60 or data collector node 50 andindicate, when nodes 10 of different network areas are expected toreport their data to the control center 60 or data collector node 50.Alternatively, the control center 60 or data collector node 50 may pollthe nodes 10 based on their location in the network.

In FIG. 5A, a street illumination system is shown, with luminaire nodes10 being distributed along the streets. Thus, when the luminaire nodes10 report their statistical data to a data collector node 50, e.g. powerstatus, dimming status, ambience brightness or the like, the datatransmission may be scheduled based on spatial or geographical grouping.Alternatively, it may be advantageous to group the luminaire nodes 10based on the road network, i.e. luminaire nodes 10 in the same streetreport their data at the same time. Preferably, luminaire nodes 10 thatare farer away from the data collector node 50 transmit their datapackets later in time than luminaire nodes 10 close to the datacollector node 50.

In a modification of the above-described embodiment, the transmissionscheduling is based on a hop-distance from the luminaire nodes 10 to thedata collector node 50. As shown in FIG. 5B, luminaire nodes 10 in awireless mesh network using multi-hop transmissions can be classified bytheir hop-distance to the data collector node 50. The hop-distancerelates to the number of hops necessary to transmit the data packet fromthe sender node 10 to the data collector node 50. For instance, aluminaire node 10 between radius 502 and radius 501 will transmit itsdata packet with two hops, whereas a luminaire node 10 within radius 501can transmit its data packet with one hop. The hop-distance or hop-countinformation may be available at the network layer of the luminaire node10. In one exemplary embodiment, data packets are transmitted from areasdefined by their hop distance to the data collector node 50 in aprogressive manner. Preferably, low hop-distance areas (e.g. withinradius 501) are scheduled to report first or are simply first polled bythe data collector node 50. By these means, it is ensured that, once theluminaire nodes 10 of a low hop-distance area have reported their datapackets, they act only as routers forwarding the data packets fromluminaire nodes 10 of higher hop-distance areas. By these means,luminaire nodes 10 in a low hop-distance area, which are normally verylikely to be overload due to the network topology, will be relieved.

Possibly, the spatial scheduling of data packet transmission based on atleast one of the location of the senders A within the network, the hopdistance of the senders A to a data collector node 50 (or networkcontroller 60) and the network addresses of the senders A is combinedwith the temporal scheduling by random selection of a time slot withinthe answer interval or by aggregating answers into one answer-batch. Bythese means, the network load can be balanced over time as well as overthe different network regions.

In a further modification of the above-described embodiment, thetransmission schedule is based on the network addresses of the luminairenodes 10. Thus, the luminaire nodes 10 may transmit their answer datapackets at different moments in time, after having received e.g. areport-request message broadcasted or multicasted by the control center60 or the data collector node 50. The broadcast or multicast message maycontain a time stamp serving as a common reference for all reportingluminaire nodes 10. Based on this reference time and a node-specific ornetwork-address-specific delay, each luminaire node 10 can determine itsindividual transmission time. This further minimizes the risk ofcollisions, while the data packets from all luminaire nodes 10 arereceived within a relatively short period of time. This is particularlyadvantageous, when the luminaire nodes 10 report status data or the likein a N:1 communication pattern to the data collector node 50 or to thecontrol center 60. In case that network addresses are distributed in adynamic fashion, a schedule table with delays specified for each networkaddress may be stored in each luminaire node 10 and the networkaddresses may be dynamically allocated to the luminaire nodes 10 basedon current requirements, thus determining the individual transmissiontime.

In a further embodiment of the present invention similar to theembodiment of transmission scheduling, back-off delays are introduced bythe MAC layer or by the network layer for retransmissions of datapackets. A back-off delay refers to a delay between subsequenttransmission attempts. For instance, when a transmission has failed, thesender node A may delay a retransmission of the data packet by theback-off delay, so that another node may complete its transmission, thusreducing a collision probability. Thus, the back-off delay may include aback-off time for medium access attempts. Here, medium access attemptrelates to the process of carrier sensing and the subsequenttransmitting or retransmitting of a data packet, when the medium isassessed to be free. Thus, the back-off time for medium access attemptsdenotes the time interval between subsequent medium access attempts. Theback-off delays are set based on at least one of the location of thenode 10 within the network, the hop-distance between the node 10 and thedata collector node 50 and the network address of the node 10.Preferably, at least one of the MAC layer and network layer back-offdelays are dependent on the distance between the node 10 and the datacollector node 50. In particular, at least one of the MAC layer back-offdelay and the network layer back-off delay becomes longer forincreasingly remote nodes 10. In this case, nodes 10 that are farer awayfrom the data collector node 50 may be forced to spread their datapackets more than nodes 10 close to the data collector node 50. By thesemeans, remote nodes 10 do not cause overload at nodes 10 that are closerto the data collector node 50. Therefore, data traffic generation of thenetwork nodes 10 is shaped such to avoid congestion near the datacollector node 50. Advantageously, this embodiment is employed togetherwith any one of the above-described embodiments.

Preferably, the above-described embodiments involving a delay of datapacket transmission are applied to time-uncritical data and/or to datatransmissions that have no stringent requirements on communication delayhomogeneity across the network. For example, such time-uncritical datamay refer to statistical data or status information reported by networknodes 10 in regular time intervals, such as consumed energy, burninghours, operation status, current or average dimming pattern, sensedambience brightness or the like. In contrast, time-critical informationmay refer to acknowledgements for control messages sent to the networknodes 10 during emergencies.

It is obvious, that the above-described embodiments can be combined invarious ways. By means of the above-described aggregating of answersinto answer-batches, local and global network load can be reduced anddata packet collisions and subsequent data losses can be avoided. Thus,routing means for a wireless mesh network are provided that allow highlyefficient and reliable data transmission, provide high scalability andfast and easy configuration and are capable of self-configuration andself-healing.

The invention claimed is:
 1. A device for controlling data packet transmissions in a wireless mesh network, wherein the mesh network comprises a plurality of nodes, each capable of communicating directly with one or more other nodes and wherein one or more of the nodes require a data packet to be transmitted from a sender node to a destination node via multi-hop transmission using multiple nodes, the device comprising: a control unit that is adapted to generate a plurality of answer data packets to more than one received data packet from more than one sender node and to aggregate said plurality of answer data packets into one answer-batch to at least one destination node; wherein redundant headers are omitted in the answer-batch to reduce data overhead.
 2. The device according to claim 1, wherein the nodes of the wireless mesh network are stationary.
 3. The device according to claim 1, wherein the received data packet is a multicast data packet sent to more than one node, wherein the answer data packet is a time-uncritical acknowledgement of the received data packet.
 4. The device according to claim 1, wherein the control unit is adapted to differentiate between time-critical and non-time critical data packets.
 5. The device according to claim 1, wherein the control unit is adapted to randomly select a time slot within a determined answer interval for transmitting an answer data packet responding to a received data packet.
 6. The device according to claim 5, wherein the answer interval is specific for the received data packet.
 7. The device according to claim 5, wherein the answer interval is determined based on one or more criteria selected from the group consisting of: number of addressee nodes of the answer, number of receiver nodes of the received data packet, network capabilities, a time stamp, information included in the received data packet, current network load, expected network load, and a stored schedule.
 8. The device according to claim 1, wherein a transmission of a data packet is scheduled based on a location of the device within the network.
 9. The device according to claim 1, wherein a transmission of a data packet is scheduled based on a hop distance of the device to a data collector node, and wherein the transmission of a data packet in an area with low hop distance to a data collector node is scheduled to be prior than the transmission of a data packet in an area with high hop distance to the data collector node.
 10. The device according to claim 1, wherein the control unit is adapted to set back-off delays based on a location of the device within the network.
 11. The device according to claim 1, wherein the control unit is adapted to postpone the answer data packets to the received data packets for aggregation in the answer-batch such that the answer data packets are transmitted within a respective answer interval.
 12. The device according to claim 1, wherein the control unit is adapted to aggregate answer data packets to an individual node.
 13. The device according to claim 1, wherein the control unit is adapted to aggregate answer data packets to a predetermined group of nodes, and wherein the control unit is adapted to determine the group of nodes based on a distance between the nodes.
 14. The device according to claim 1, wherein the device is used in telemanagement of a lighting system for controlling lighting functions of luminaire nodes.
 15. The device according to claim 1, wherein the received data packet is a request for data delivery, and wherein the answer data packet is a time-uncritical data packet.
 16. The device according to claim 1, wherein a transmission of a data packet is scheduled based on a network address.
 17. A system for controlling data packet transmission in a wireless mesh network, the system comprising: a plurality of nodes, wherein the nodes of the wireless mesh network are stationary; and at least one data collector node; wherein at least one of the nodes comprises a device according to claim 1, wherein the data collector node is adapted to multicast a data request message to at least some of the nodes, to which the nodes answer with a node-specific delay after having received the data request message.
 18. The system according to claim 17, wherein at least one data collector comprises a device according to claim 1, and wherein the at least one data collector node is adapted to poll at least one of the nodes for data packet transmission based on a location of the node.
 19. The system according to claim 17, wherein at least one data collector comprises a device according to claim 1, and wherein the at least one data collector node is adapted to poll at least one of the nodes for data packet transmission based on based on a hop distance of the node to the data collector node.
 20. A method for controlling data packet transmission in a wireless mesh network, wherein the mesh network comprises a plurality of nodes, each capable of communicating directly with one or more other nodes and wherein one or more of the nodes require a data packet to be transmitted from a sender node to a destination node via one or more intermediate nodes, the method comprising the steps of: generating a plurality of answer data packets to more than one received data packet from more than one sender node, and aggregating said plurality of answer data packets into one answer-batch to at least one destination node; and, wherein redundant headers are omitted in the answer-batch to reduce data overhead. 